The invention relates to a method and a wind sifter for grading comminuted charging material (sifting material).
Methods and wind sifters of the type required are known in various forms from practice. Those wind sifters are so-called dynamic wind sifters in which an approximately basket-form sifting rotor, whose outer circumference is equipped with sifting strips or the like, is generally rotatably supported and driven inside a sifter housing. A substantially annular sifting chamber which is generally also surrounded by a vane ring radially towards the outside is formed above all in the region of the outer circumference, that is to say, around the sifting rotor. A tubular withdrawal duct for sifting air charged with fine material adjoins at least one end face of the sifting rotor which is generally rotating about a horizontal or upright axis. Sifting air and charging material are introduced into the sifting chamber in a suitable manner.
While the heavier coarse material falls out substantially downwards and is drawn off by way of a suitable collecting chamber for coarse material, the fine material is removed together with the sifting air first through the circumference of the rotor and into the inside of the sifting rotor and then from there through at least one end face into the withdrawal duct, while applying an appropriate partial vacuum, and from there into a suitable separating or filtering device. The fineness of the fine material which is discharged with the stream of sifting air, and which can be drawn off as fine material, can be adjusted within a suitably large range by the speed of the sifting rotor and/or by regulation of the amount of sifting air.
In sifting methods and wind sifters of the above-mentioned type, there are also provided, in the transition region from the appropriate end face of the sifting rotor to the stationary withdrawal duct, annular-blocking-air sealing regions into which blocking air is introduced in such an amount and under such a pressure that a bypass stream from the sifting chamber into the withdrawal duct can be suppressed as far as possible, and preferably completely. Thus the intention is quite deliberately to prevent portions of coarse grain or grit from passing with the bypass stream into the withdrawal duct for sifting air and fine material in order thereby to ensure that the fine material has a relatively high degree of fineness.
In the manufacture of cement from cement clinker, granulated blast-furnace slag and the like, the comminution of those starting materials has increasingly involved a transition to energy-saving grinding methods or grinding systems in which the comminution work is carried out above all in material bed cylinder crushers and roller mills, or bowl mill crushers, downstream of which are arranged dynamic heavy-duty sifters. Admittedly, in that method of grinding cement, the aim is, inter alia, to obtain relatively high product fineness; however, with those product fineness, specific quality properties of the cement are to be observed. This also includes the amount of water required to achieve the standard consistency, that is to say, an increased addition of water to the so-called standard stiffness corresponds as a rule to an increased water requirement on the part of the concrete.
In this connection, it is already known that cements that are ground with grinding systems or grinding methods containing a material bed cylinder crusher or a roller mill require more water than do cements that are produced, for example, in grinding systems having ball mills. The consequence of that higher water requirement (higher water-cement ratio) is a higher pore volume of the cement mortar which leads to a lower standard strength of the cement-mortar prisms. This undesirably higher water requirement for cements that are ground in grinding systems having material bed cylinder crushers or roller mills is basically attributed to a relatively narrow grain size distribution.
The grain size distribution in cement and cement-like products is normally presented as a cumulative mass distribution in the so-called xe2x80x9cRRSB diagramxe2x80x9d (after Rosin, Rammler et al.) in which the axis scales are so selected that the cumulative mass distributions of normal mineral comminution products appear in the form of straight lines, the cumulative distributions being described by two parameters, namely by the slope n and the parameter of position d"", the slope n indicating at what angle a mean straight line extends (for example n=tan xcex1=1.0=45xc2x0), that is to say, the greater the slope n, the steeper the curve characteristic in the RRSB diagram, while the parameter of position d"" fixes the grain size in xcexcm on the mean straight line at a specific screening residue (residue value) of 36.8%.
The fineness of cement, blast furnace powder or the like is usually indicated as a mass-related surface in Blaine (cm2/g). The higher that degree of fineness is, the greater are also the strengths of concretes and mortars produced therefrom. Depending on the use for which a cement is intended, the grain fineness and the water absorption capacity of the cement, and thus the strength and workability, must be in accordance with one another.
In order, in grinding methods and grinding systems in which material bed cylinder crushers or roller mills are used for comminution and dynamic heavy-duty sifters are used for grading, to be able to achieve a sufficiently broad grain size distribution, or a sufficiently large grain band width (by an appropriate slope n in the RRSB diagram), even with relatively high degrees of fineness of the end product (fine material), it has also already been proposed to convey the comminuted charging material (sifting material) to be graded to a grading stage comprising at least two grading units which are adjusted to different degrees of fineness and to each of which selectable partial streams of the charging material are conveyed and whose fine material portions are mixed with one another.
Thus the end product (fine material as a whole) is also to be adapted in respect of its processing behavior and its strength development to the quality standard which is produced in grinding systems having ball mills (see EP-A-0 406 591). However, it must be accepted that at least some of the energy saving gained in the comminution of the starting materials will be lost again through the amount of equipment used in grading the comminuted charging material (for example, grading stage having at least two grading units).
It is, therefore, an object of the present invention to provide an improved sifting method and a wind sifter using the sifting method in which the grain size distribution and therefore the grain band width in the fine material, or the end product, can be adjusted reliably and with relatively low structural expenditure, with sufficiently great leeway.
An important concept of the present invention is regarded as being the fact that the grain size distribution in the discharged fine material, which can be drawn off as the end product, is adjusted with the aid of a bypass stream, which flows in an alterable volume from the sifting chamber into the withdrawal duct (for the mixture of sifting air and fine material) and which is charged with sifter charging material or sifter grit (bypass material), by controlling the volume of that bypass stream by very selective adjustment of the blocking air supply in terms of its amount.
Therefore, whereas in the known construction described in the introduction (DE-A-195 05 466) the blocking air is blown into the ring seal region between the sifter rotor and the stationary withdrawal duct quite deliberately for the sole purpose of preferably preventing any bypass stream and thus any spray grain (that is to say, sifter charging material or sifter grit) from passing into the withdrawal duct for the mixture of sifting air and fine material, according to the present invention the blocking air supply is used to ensure that an adjustable portion of the bypass stream charged with the so-called bypass material is introduced very selectively into the mixture of sifting air and fine material in the withdrawal duct in such an amount that the grain size distribution range of the fine material, or end product, can thereby be controlled in the necessary manner.
This control of the grain size distribution range (grain band width) in the fine material can be carried out extremely reliably and reproducibly. Since, for this control, only the blocking air supply has to be adjusted in terms of its amount and/or pressure, the grading operation can be carried out in a single wind sifter, that is to say, with regard to the known construction according to EP-A-0 406 591, the arrangement of at least two grading units, or wind sifters, connected in parallel can be dispensed with, which means a substantial reduction in expenditure on equipment.
The method according to the invention can be used very especially advantageously and very selectively in the case of high product fineness, it nevertheless being possible to adjust an optimum grain size distribution, or distribution range, for the particular use for which the fine material, or end product, is intended, that is to say, an optimum compromise can also be achieved with respect to the quality properties of the product, that is to say, between the standard compressive strength and the workability of the end product, for example cement.
In the tests on which the invention is based, it was found to be very advantageous if the amount of blocking air supplied was adjustable in a range of approximately from 5 to 25%, preferably approximately from 10 to 20%, of the amount of sifting air supplied to the sifting chamber. Advantageously, a smaller amount of blocking air is adjusted if the fine material has a relatively low degree of fineness and a larger amount of blocking air is adjusted if the fine material (end product) has a relatively high degree of fineness.
Those tests achieved, for example, with a proportion of blocking air of approximately 10%, a xe2x80x94relatively lowxe2x80x94 product fineness in the case of cement or granulated blast-furnace sand of approximately 3000 Blaine (cm2/g) with a parameter of position d"" (in the RRSB diagram) of approximately from 16 to 20 xcexcm, whereas, with a proportion of blocking air of approximately 20%, a relatively high product fineness was achieved, for example, for cement or granulated blast-furnace sand, with approximately 5000 Blaine (cm2/g) with a parameter of position d"" of approximately from 8 to 12 xcexcm, the larger amount of blocking air (higher product fineness) leading to a higher slope n than in the case of the lower proportion of blocking air (for relatively low product fineness).
According to a further form of the invention, it is considered advantageous if the pressure level of the blocking air supplied is controlled in accordance with the load on the sifter and the static pressure in the withdrawal duct (behind the sifting rotor) for the mixture of sifting air and fine material. It is thus possible to ensure in a suitable and adequate manner that the pressure loss in the withdrawal duct is overcome by the supplied blocking air, or amount of blocking air. It should be mentioned in this connection that the pressure loss in the withdrawal duct depends above all on the charge of fine material in the sifting air, the amount of sifting air, the rotor speed, and the like, a high charge, a large amount of sifting air and a high rotor speed resulting in a high pressure loss and vice versa.
A wind sifter in a form according to the invention is distinguished by the fact that a control device co-operating with the blocking air supply is provided to adjust the grain size distribution in the discharged fine material, a bypass guide connecting the sifting chamber to the withdrawal duct by way of the ring seal and the blocking air supply connected to the ring seal being adjustable in respect of the pressure and/or the amount of the blocking air in such a manner that a bypass stream leaving the sifting chamber and charged with spray grain can be introduced in a controllable volume through the bypass guide into the withdrawal duct for the mixture of sifting air and fine material.